After doing a a completely fresh install of 18.2. Cinnamon, I wanted to change my user login window image. To me this is kind of minor, but I went to change the desktop background screen image (I used my own image not the Mint ones). After updating the system and rebooted the OS, the Plymouth login screen no longer showed up. In the System Settings I opened up the Login Window dialog window and tried to put my own images in there. After rebooting the login window was still black, not the image I wanted. I did change my desktop screen image back to the Mint image and when I went to reboot it, the original login image came back.
Just one more thing, I did attempt, as root, to put my image in the /usr/share/backgrounds/linuxmint directory, but my image did not show up in the System Settings/ Login Window dialog. I was wondering how can I use my one images for the login screen?
From what I am doing it seems that if I use my own images, the login window doesn`t work using my images. (I`ve tried both .png and .jpg files).

I had a similar problem. I suspect that your image file is not accessible to whatever linux user is running the login screen - check your file permissions to ensure that the file can be read by other users, also check the containing folder tree to ensure that their content can be accessed by other users.

First, maybe you should put your file in /usr/share/backgrounds/linuxmint-sonya, does that help?
Strange thing is that the dialog looks for diskcopy-files...

If you want a pretty customizable login screen, you have to install MDM, the older loginscreen-manager (you can install it from the softwaremanager); Cinnamon 18 only has LightDM installed as loginscreen-manager, which is poorly customizable.
After installing MDM, there are two instances of Menu>Management>loginscreen (not sure if I translated menu-options correctly, as I run Dutch version of Cinnamon), try both to find the MDM-dialog. But this offers no changing background image.

To change background image in MDM (at your own risk, however!!):
1- First look for a preferred theme in the MDM-dialog
2- Go as root to /usr/share/mdm/themes
3- Copy the folder of your preferred theme to a save location (eg desktop), and rename it (eg Plymouth). Open that folder
4- Put your *.png-image in it (eg Plymouth.png)
5- Open theme.xml with leafpad (no-nonsense text-editor; you can install it from the softwaremanager)
6- Under <item type="pixmap">, point to your imagefile (eg <normal file="Plymouth.png"/>)
7- Save changes to the theme.xml file and close it, do not yet close your folder
8- Open now GdmGreeterTheme.desktop with leafpad and change Name (eg Name=Plymouth)
8- Copy your renamed folder to /usr/share/mdm/themes (opened as root!)
9- If everything is good, you can find your theme in the MDM-dialog

Cinnamon 18 only has LightDM installed as loginscreen-manager, which is poorly customizable

Not quite right.
There is no Cinnamon 18, yet, very likely you mean Linux Mint 18 Cinnamon.
Linux Mint 18 and 18.1 bring along MDM still.
LightDM was introduced by Mint 18.2 (Cf. here: https://linuxmint.com/rel_sonya_xfce_whatsnew.php)
The login managers MDM and LightDM are not specific to the desktop environments Cinnamon, Mate or xfce. They are common to all.

Cinnamon 18 only has LightDM installed as loginscreen-manager, which is poorly customizable

Not quite right.
There is no Cinnamon 18, yet, very likely you mean Linux Mint 18 Cinnamon.
Linux Mint 18 and 18.1 bring along MDM still.
LightDM was introduced by Mint 18.2 (Cf. here: https://linuxmint.com/rel_sonya_xfce_whatsnew.php)
The login managers MDM and LightDM are not specific to the desktop environments Cinnamon, Mate or xfce. They are common to all.

Yes I meant Linux Mint 18 Cinnamon.
I recently installed more instances of LM (18.2) Cinnamon NL-version and there I had to search for MDM, it happened to be missed out; had to install it.

Note, that the development for mdm has been stopped some time ago. Since Mint 18.2 lightdm is used by default. Especially you have to expect possible problems with auto-login, which will very most likely never get solved.